Landing method and system for air vehicles

ABSTRACT

Methods are provided for operating an air vehicle, the air vehicle including fixed wings configured to provide mild stall characteristics including a post-stall regime, and a propulsion system capable of generating a controllable thrust, the thrust being variable at least between an idle thrust and a maximum thrust. During a landing maneuver, the air vehicle is caused to attain an angle of attack corresponding to said post-stall regime, and during the landing maneuver, there is concurrently generated a thrust level of said thrust greater than said idle thrust to provide a thrust vector having a thrust lift force component at landing. Corresponding control systems are also provided, and air vehicles including such control systems are also provided.

TECHNOLOGICAL FIELD

The presently disclosed subject matter relates to landing methods and landing systems for air vehicles.

PRIOR ART

References considered to be relevant as background to the presently disclosed subject matter are listed below:

-   -   1. U.S. Pat. No. 8,109,473     -   2. Nagel, A., and Shepshelovich, M., “Development of High-Lift         UAV Wings”, 24^(th) AIAA Applied Aerodynamics Conference, San         Francisco, Calif., 5-8 Jun., 2006.     -   3. Shepshelovich, M., “The Progress in Development of UAV         Wings”, International Conference ICAUV-2009, Bangalore, India,         2009     -   4. Steinbuch, M., Marcus, B., Shepshelovich, M., “Development of         UAV Wings—Subsonic Designs”, 41^(st) Aerospace Sciences Meeting,         Reno, Nev., 6-9 Jan., 2003.     -   5. McCormick, B. W., “Aerodynamics Aeronautics and Flight         Mechanics”, John Wiley & Sons, book, 2^(nd) edition, 1995.     -   6. U.S. Pat. No. 7,992,827     -   7. Abbott, I. H. and Von Doenhoff, A. E., “Theory of Wing         Sections”, Dover Publication, book, 1959.     -   8. FR 53,254     -   9. GB 521,190

Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

BACKGROUND

Conventional horizontal landing of a fixed-wing air vehicle (having a forward speed sufficient to sustain aerodynamic flight), particularly where the forward thrust vector is fixed with respect to the air vehicle, is conventionally accompanied by an increase in lift and reduction in air speed. Where the air vehicle is a powered air vehicle, landing is also conventionally accompanied by a reduction in engine power output, and the engine is often at idle during the landing maneuver until at least touchdown, to reduce airspeed and to provide a reasonable glide angle. While in some air vehicles, the engine power can be significantly increased after touchdown to provide reverse thrust to further reduce the forward speed of the air vehicle, maintaining a forward thrust after idle setting conventionally adversely affects landing performance.

In addition, conventional horizontal landings are subject to the conventional speed safety constraint, in which the airspeed of the air vehicle is conventionally maintained at least at 20% higher than the respective stall speed, as long as the air vehicle is airborne. Furthermore, conventional horizontal landings are often accompanied with the deployment of high lift devices and the use of drag inducing devices to provide increased glide angles.

Some types of air vehicles have wings which exhibit mild-stall characteristics at high lift, and are at least partly based on a two-element aerofoil design. In this connection, two-element slotted aerofoils are well known, and are disclosed for example in U.S. Pat. No. 8,109,473 and U.S. Pat. No. 7,992,827, assigned to the present assignee.

Slotted aerofoils are two-element aerofoils composed of a first element (the main aerofoil body), and a second element (for example in the form of a flap or aileron). The second element is separated from the first element by a slot which is open for the airflow at any deflection of the second element. In one class of such slotted aerofoils, the slot is permanent, and facilitates actuation of the second element through positive or negative deflection angles. Such slotted aerofoils can be designed for cruising/loitering flight at high lift coefficients, and rely on a second element rotation around an external fixed hinge point for adjustment of the aerofoil to different flight regimes. There are also cases in which the second element is rigidly connected with respect to the first element, providing a mechanically simple system with high maximum lift characteristics.

In general, stall characteristics of such aerofoils tend to deteriorate as maximum lift is increased, resulting in more difficulties when attempting to comply with considerations of flight safety (complying with the speed safety constraint) and to avoid unfavorable stall patterns. This is especially relevant for high-lift, long endurance wings of some UAV's, such as for example the Heron high-lift long endurance UAV, manufactured by IAI, Israel. In the Heron UAV, optimum endurance performance is achieved via high loitering lift coefficients, which requires high maximum lift.

However, in some cases, such slotted aerofoils are configured to provide mild-stall characteristics at high lift, resulting in a smooth plateau of lift coefficients at post-stall angles of attack. Specifically, mild stall is characterized by almost constant level of the lift at post-stall domain and is associated with slowly creeping trailing edge separation that moderates the rate of lift losses at high angles of attack, typically resulting in an approximately constant lift coefficient (up to about 5% of the maximum lift coefficient for at least about 7° after the stall angle of attack, or within between about 5% and about 10% of the maximum lift coefficient for at least about 5° after the stall angle of attack).

The stall angle of attack can be defined as the angle of attack at which maximum lift coefficient (or up to about 95% or 96% or 97% or 98% or 99% of maximum lift coefficient) is first realized. The stalling speed is dependent on the weight (W) of the air vehicle, maximum lift coefficient (C_(L max)), wing area (S_(w)), and air density (ρ), and is generally defined as:

V _(stall)=(2W/(ρ*C _(L max) *S _(w)))^(0.5)

For example, mild-stall characteristics at an extended range of post-angle angles of attack can be achieved relying on the concept of the so-called mild stall ramp (MS-ramp) at the aft portion of the upper surface of the main body (see for example U.S. Pat. No. 8,109,473 (ref 1 above), Nagel et al (Ref 2 above: “Development of High-Lift UAV Wings”, 24^(th) AIAA Applied Aerodynamics Conference, San Francisco, Calif., 5-8 Jun., 2006), and Shepshelovich (Ref 3 above: “The Progress in Development of UAV Wings”, International Conference ICAUV-2009, Bangalore, India, 2009)).

Such conventional two-element aerofoils also provide a longitudinal overlap (i.e., generally defined as an overlap in a direction generally parallel to the reference line of the aerofoil), between the trailing edge of the primary element (also referred to interchangeably herein as the trailing end of the primary element) and the leading edge of the secondary element (also referred to interchangeably herein as the leading end of the secondary element). The longitudinal overlap is positive when part of the primary element is superposed over part of the secondary element (i.e., when viewed in a direction normal to the reference line of the aerofoil, for example), and the positive direction is in a direction towards the leading edge of the aerofoil. Conversely, the longitudinal overlap is negative when there is no such superposition, and the trailing end of the primary element is longitudinally spaced away from the leading end of the secondary element (i.e., when viewed in a direction normal to the reference line of the aerofoil, for example), and the negative direction is in the direction towards the trailing edge of the aerofoil. The aforesaid “longitudinal overlap” is also referred to herein interchangeably as “overlap” or “axial overlap”.

Similarly, a gap can also be defined for the two-element aerofoils, as the “vertical” spacing between the trailing end of the primary element and the secondary element, i.e., in a direction generally orthogonal to the direction of the longitudinal overlap, i.e., in a direction generally orthogonal to the reference line of the aerofoil. Alternatively, the gap can be defined in a direction parallel to the thickness of the aerofoil.

Referring to FIG. 1, Abbott et al (Ref 7 above: “Theory of Wing Sections”, Dover Publication, book, 1959”) provides a contour map of maximum lift coefficient as a function of slot overlap and gap. The contours show that maximum lift coefficient for the NACA 23012 wing section was obtained for a longitudinal overlap of about 1% and gap of about 1.2%. As the longitudinal overlap is increased positively (i.e., in the positive direction, towards the aerofoil leading edge), for example to +2%, the maximum lift coefficient drops to about 2.75, and as the longitudinal overlap is increased negatively (i.e., in the negative direction, towards the aerofoil trailing edge), for example to 0%, the maximum lift coefficient again drops to about 2.75. Similar effects are observed as the gap is increased or decreased from about just about 1.2%. It is to be noted that the particular form of the contour map is aerofoil-dependent as well as Reynolds number-dependent.

Thus, conventionally, two-element slotted aerofoil design seeks to optimize performance by providing a positive longitudinal overlap close to zero, typically between 0° to 3° for example, for the range of flap angles δ in which maximum lift is to be maximized. For example the range of flap angle δ can be between about 25° and 30°, often required for take-off and landing performance, for example, and generally in which the flow over the secondary element is fully attached. Accordingly it follows that, if the second element of such two-element slotted aerofoils is further deflected to flap angles δ associated with use as airbrake (wherein generally, the flow over the secondary element is fully detached to maximize drag, for example flap angle δ between about 60° and 80°) about a fixed hinge point, the resulting overlap becomes negative, as illustrated in FIGS. 2(a) and 2(b), for example. Concurrently, the lift coefficient drops rapidly as flap angle δ increases past about 30° and continues dropping steadily past 60° (FIG. 3).

Indeed, while conventional airbrakes for air vehicles are designed to provide high levels of drag via large deflections of the airbrakes, such high levels of drag are conventionally coupled with a significant drop in maximum lift characteristics at these high deflections. Thus, while maximum lift of high-lift devices is realized with fully attached flow on the deflected flap segments (i.e., at conventional positive flap deflections), such flap segments when used as airbrakes (i.e., at airbrake flap deflections) rely on fully separated flow on the flap segment for producing maximum possible drag for deployed airbrake (δ_(airbrake)˜+60° to +80°).

Such a conventional airbrake concept is described for example in Steinbuch et al (Ref 4 above: “Development of UAV Wings—Subsonic Designs”, 41^(st) Aerospace Sciences Meeting, Reno, Nev., 6-9 Jan., 2003), and was adopted in the development of the aforementioned IAI Heron UAV.

Providing a positive longitudinal overlap to maximize the maximum lift coefficient at flap angles associated with maximum lift conditions has become the conventional principle for design of high-lift devices (see, for example, the cases presented in McCormick (Ref. 5 above: “Aerodynamics Aeronautics and Flight Mechanics”, John Wiley & Sons, book, 2^(nd) edition, 1995). Such high-lift devices may be of single-slotted flap type, for example. Conventionally, the optimum lift performance of these devices is achieved for positive overlap, or overlap close to zero (between −1.0 and 3.0% of chord) at flap deflections associated with maximum lift and almost fully attached flow on the high lift devices, and at flap deflections not greater than about 30°.

It is to be noted that comparatively mechanically simple designs for high-lift devices, for example as provided on at least some IAI high-lift, two-element wings, rely on single-slotted flap with a fixed hinge point location. In such cases, the maximum positive conventional flap deflections with fully attached flow are around δ˜+20° to +30°, depending on the flight Reynolds numbers (for example Reynolds numbers between about 0.2*10⁶ to about 2.0*10⁶), providing for maximizing the maximum lift coefficient. In such cases, flap deflections above δ˜+30° require a simple continuation of the flap rotation around the fixed hinge point, when the flap segment becomes fully separated and begins to operate in airbrake mode. In such cases, this continuation of the flap rotation around the fixed hinge point to large deflections angles above δ˜30°, and including 60° to 80°, or to more than 80°, automatically results in a longitudinal overlap that rapidly increases negatively, for example −3.5% of the chord at δ˜60°.

GENERAL DESCRIPTION

The term “suction peak” refers herein to relatively fast flow acceleration at the leading edge of the aerofoil, producing a spiky pressure distribution. The term “blunt leading edge” refers herein to increased thickness and increased local radius of the forward portion of the aerofoil that prevents formation of suction peak at high angles of attack and produces a rounded pressure distribution at the leading edge of the aerofoil.

Herein, the term “mild-stall aerofoils” (MS-aerofoils) refers to a class of wing sections aerofoils characterized by the lift curve having a plateau range of lift coefficients at post-stall angles of attack followed by gradual decline of the lift at high post-stall angles of attack. The plateau range and the gradual decline correspond to a gradually developing trailing edge separation of the flow, until total separation of the aerofoil is achieved. Accordingly, it is often possible to more or less maintain the lift generated by the wing at the level of maximum lift, or within about 10% for example, for a relatively wide range of post-stall angles of attack, the range being typically at least about 5°, 7° or 8° or greater than 8°.

Without being subject to theory, the mechanism of mild stall of high-lift aerofoils can be explained by the combination of continuous lift build-up at the forward portion of MS-aerofoils and slowly creeping trailing edge separation. The blunt or rounded leading edge of MS-aerofoils helps to produce rounded pressure distributions at the forward portion of the wing section, preventing formation of sharp suction peak at stall and post-stall angles of attack. This supports the gradual development of trailing edge separation that is initiated prior to the stall angle of attack and is controlled by the shape of aft camber.

Herein, the term “High-Lift Mild-Stall aerofoils” (HL-MS aerofoils) refers to a special class of mild stall aerofoils that, for any Reynolds number in the range of between about 0.3*10⁶ to about 2*10⁶, provide maximum lift C_(l max) above the C_(l max) for the same Reynolds number according to a minimum boundary line as defined by a linear variation between points of C_(l max)=1.6 at Re=0.3*10⁶ and C_(l max)=1.95 at Re=2.0*10⁶. This variation serves to differentiate HL-MS aerofoils (providing high lift) from conventional mild stall aerofoils (MS-aerofoils) with moderate maximum lift, wherein conventional mild stall aerofoils, such as for example NACA4415 and its derivatives, achieve maximum lift coefficient below this minimum boundary.

According to a first aspect of the presently disclosed subject matter there is provided a method for operating an air vehicle, the air vehicle comprising fixed wings configured to provide mild stall characteristics including a post-stall regime, and a propulsion system capable of generating a controllable thrust, the thrust being variable at least between an idle thrust and a maximum thrust, the method comprising:

-   -   during a landing maneuver, causing the air vehicle to attain an         angle of attack corresponding to said post-stall regime; and     -   during said landing maneuver, concurrently generating a thrust         level of said thrust greater than said idle thrust to provide a         thrust vector having a thrust lift force component at landing.

For example, the method further comprises, at least during said landing maneuver, generating significantly elevated drag levels as compared with nominal drag levels prior to said landing maneuver.

Additionally or alternatively, for example, said angle of attack is greater than 10°.

Additionally or alternatively, for example, said angle of attack is between 10° and 20°.

For example, said thrust lift force component is between 10% and 50% of said thrust level.

Additionally or alternatively, for example, said thrust level is between 10% and 50% of said maximum thrust.

Additionally or alternatively, for example, in said landing maneuver, the air vehicle is in a downward glide path defined by a glide angle. For example, said glide angle is in the range of between −3° to about −15°.

Additionally or alternatively, for example, said thrust vector is at a pitch angle (θ) to the horizontal, wherein said pitch angle (θ) is related to said angle of attack (α) and said glide angle (γ) by the expression:

α=θ−γ

For example said thrust lift force component (TL) is related to said thrust level (T) and said thrust vector pitch angle (θ) by the expression:

TL=T*sin θ

Additionally or alternatively, for example, said thrust vector has a forward thrust component (TH) at landing, wherein said forward thrust component (TH) is related to said thrust level (T) and said thrust vector pitch angle (θ) by the expression:

TH=T*cos θ

Additionally or alternatively, for example, said glide angle (γ) is related to said thrust level (T) and said elevated drag (D) by the expression:

γ=(D−T)/L

Additionally or alternatively, for example, the method comprises operating the air vehicle during said landing maneuver at airspeeds below the corresponding speed safety margin, while the air vehicle is airborne.

Additionally or alternatively, for example, the method comprises operating the air vehicle during said landing maneuver at airspeeds not exceeding the corresponding stall airspeed of the air vehicle, while the air vehicle is airborne.

Additionally or alternatively, for example, the method comprises operating the air vehicle during said landing maneuver at airspeeds between 90% and 95% of the corresponding stall airspeed of the air vehicle, while the air vehicle is airborne.

Additionally or alternatively, for example, said air vehicle is configured for providing said elevated drag levels while not significantly degrading aerodynamic lift provided by said wings during said landing maneuver.

According to this aspect, and according to other aspects, of the presently disclosed subject matter the air vehicle is provided with lift generating wings having at least some aerofoil cross-sections thereof comprising a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, wherein at least for said design airbrake deflection angle said axial overlap is numerically greater than −0.5% of the aerofoil chord.

For example, the design airbrake deflection angle is greater than 40°.

Additionally or alternatively, said design airbrake deflection angle is greater than 45°.

Additionally or alternatively, said design airbrake deflection angle is greater than 55°.

Additionally or alternatively, said airbrake drag corresponds to an airbrake aerofoil drag coefficient that is at least 150% greater than a datum drag coefficient of the aerofoil at a zero said flap deflection angle. For example, said airbrake aerofoil drag coefficient is greater than 0.15. Additionally or alternatively, for example, said airbrake aerofoil drag coefficient is greater than 0.2. Additionally or alternatively, for example, said airbrake aerofoil drag coefficient is greater than 0.3.

Additionally or alternatively, the primary element is configured for providing high lift mild stall characteristics, and wherein the aerofoil is configured to generate said airbrake drag while concurrently retaining said mild stall characteristics.

Additionally or alternatively, at least for said design airbrake deflection angle said axial overlap is between −0.5% and +4% of the aerofoil chord.

Additionally or alternatively, said axial overlap provides a smooth variation of aerofoil maximum lift coefficient with said flap deflection angle for a range of said flap deflection angles at least ±10° from said design airbrake deflection angle.

Additionally or alternatively, at said axial overlap, the aerofoil maximum lift coefficient is maintained constant within 0.1 with said flap deflection angle for a range of said flap deflection angles at least ±10° from said design airbrake deflection angle.

Additionally or alternatively, a value for said axial overlap is chosen to maximize the maximum lift coefficient obtained for the aerofoil at said design airbrake deflection angle.

Additionally or alternatively, said secondary element is in the form of a slotted flap.

Additionally or alternatively, the secondary element is deflectable with respect to the primary element about said fixed hinge point to provide said flap deflection angle ranging between about −15° and +80°.

Additionally or alternatively, at said design airbrake deflection angle, at least a majority of an airflow over said secondary element is fully detached.

Additionally or alternatively, at said design airbrake deflection angle said aerofoil generates a maximum lift coefficient of about 2.5 or greater than 2.5.

Additionally or alternatively, at said design airbrake deflection angle corresponds to a design Reynolds number of between 0.3*10⁶ and 1.0*10⁶.

Additionally or alternatively, at said design airbrake deflection angle corresponds to a design Reynolds number of 0.4*10⁶.

Additionally or alternatively, for example, the air vehicle is a UAV.

Additionally or alternatively, for example, the air vehicle has a weight between about 50 kg to about 500 kg.

According to the first aspect of the presently disclosed subject matter there is also provided a control system for operating an air vehicle, the air vehicle comprising fixed wings configured to provide mild stall characteristics including a post-stall regime, and a propulsion system capable of generating a controllable thrust, the thrust being variable at least between an idle thrust and a maximum thrust, the control system comprising a controller configured for:

-   -   causing the air vehicle to attain an angle of attack         corresponding to said post-stall regime during a landing         maneuver; and     -   concurrently generating during said landing maneuver, a thrust         level of said thrust greater than said idle thrust to provide a         thrust vector having a thrust lift force component at landing.

For example, the control system is configured for operating according to the method defined according to the first aspect of the presently disclosed subject matter.

According to the first aspect of the presently disclosed subject matter there is also provided an air vehicle, comprising the control system as defined according to the first aspect of the presently disclosed subject matter.

According to this aspect of the presently disclosed subject matter the air vehicle is optionally further provided with lift generating wings having at least some aerofoil cross-sections thereof comprising a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, wherein a value for said axial overlap is chosen to maximize the maximum lift coefficient obtained for the aerofoil at said design airbrake deflection angle.

According to a second aspect of the presently disclosed subject matter there is provided a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, wherein a value for said axial overlap is chosen to maximize the maximum lift coefficient obtained for the aerofoil at said design airbrake deflection angle.

According to a third aspect of the presently disclosed subject matter there is provided a method for landing an air vehicle, the air vehicle comprising wings having aerofoil sections corresponding to the aerofoil as defined above for the first or second aspects of the presently disclosed subject matter, the method comprising maximizing glide angle by:

-   -   deflecting the secondary element to said design airbrake         deflection angle to provide said airbrake drag;     -   reducing airspeed of the air vehicle to values corresponding to         the level of maximum lift.

For example, the aerofoil is configured to generate large values of drag concurrently with high lift to maintain low values of the lift-to-drag ratio (L/D), and thereby provide high values of glide angle γ.

For example, said glide angle γ is greater than 1.5°, or greater than 5°, or greater than 12°, or greater than 15°. For example, said glide angle γ is in the range between about 3° and about 15°. For example, said glide angle γ is in the range between 10° and about 15°.

Additionally or alternatively, said glide angle γ is maximized to minimize exposure time of the air vehicle during landing thereof.

Additionally or alternatively, said glide angle γ is chosen to compensate for tall obstacles are located in the vicinity or perimeter of the landing site for the air vehicle.

Additionally or alternatively, said glide angle γ is controllably selected during landing to provide a desired precision for the touchdown point.

Additionally or alternatively, the aerofoil is configured to provide high-lift mild stall characteristics, and wherein prior to landing the air vehicle is flown with an absence of a conventional speed safety margin.

Additionally or alternatively, the aerofoil is configured to provide high-lift mild stall characteristics, and wherein prior to landing the air vehicle is flown at post-stall angles of attack maintaining high levels of lift.

According to a fourth aspect of the presently disclosed subject matter there is provided a method for operating an air vehicle, the air vehicle comprising wings having aerofoil sections corresponding to the aerofoil as defined above for the first or second aspects of the presently disclosed subject matter, wherein the aerofoil is configured to provide high-lift mild stall characteristics, the method comprising flying the air vehicle with an absence of a conventional speed safety margin.

According to a fifth aspect of the presently disclosed subject matter there is provided a method for operating an air vehicle, the air vehicle comprising wings having aerofoil sections corresponding to the aerofoil as defined above for the first or second aspects of the presently disclosed subject matter, wherein the aerofoil is configured to provide high-lift mild stall characteristics, the method comprising flying the air vehicle at post-stall angles of attack maintaining high levels of lift.

According to a sixth aspect of the presently disclosed subject matter there is provided a method for designing a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, the method comprising:

at least for said design airbrake deflection angle, setting said axial overlap to be numerically greater than −0.5% of the aerofoil chord.

According to a seventh aspect of the presently disclosed subject matter there is provided a method for designing a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, the method comprising defining a value for said axial overlap such as to maximize the maximum lift coefficient obtained for the aerofoil at said design airbrake deflection angle.

A feature of at least some examples of the presently disclosed subject matter is that the landing method can be beneficial to at least some small and/or medium sized tactical UAV (i.e., having a weight between about 50 Kg to about 500 Kg).

A feature of at least some examples of the presently disclosed subject matter is that the landing method can be beneficial for operating from unprepared or semi prepared airfields, and can be useful in avoiding ground obstacles at approach to landing, particularly for at least some small and/or medium sized tactical UAV.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it can be carried out in practice, examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates contours of flap location for maximum lift coefficient provided with a slotted flap on a NACA 23012 wing section.

FIG. 2(a) and FIG. 2(b) illustrate the effect of flap rotation on axial overlap for a baseline aerofoil SA-21.

FIG. 3 illustrates the variation of maximum lift coefficient with flap angle for baseline aerofoil SA-21, and the discontinuous change in the variation at flap angle of about 30° to 35°.

FIG. 4 illustrates in plan view an air vehicle comprising wings based at least in part on two-element aerofoils according to at least one example of the presently disclosed subject matter.

FIG. 5 illustrates a two-element aerofoil according to one example of the presently disclosed subject matter.

FIGS. 6(a) to 6(e) illustrate various modes of operation of the aerofoil of FIG. 5 corresponding to respective flap deflections.

FIGS. 7(a) to 7(c) illustrate the axial overlap obtained with the aerofoil of FIG. 5 corresponding to flap deflections of 0°, +25°, +60°, respectively.

FIGS. 8(a) and 8(b) illustrate, at Reynolds numbers of 0.3*10⁶ and 0.5*10⁶, respectively, lift coefficient variations with angle of attack obtained with the aerofoil of FIG. 5 corresponding to flap deflections of 0°, +10°, +20°.

FIGS. 9(a) to 9(d) illustrate pressure distributions obtained in wind tunnel tests with the aerofoil of FIG. 5 at a number of post-stall angles of attack, corresponding to flap deflection of 0° and Reynolds numbers of 0.3*10⁶.

FIGS. 10(a) to 10(d) illustrate pressure distributions obtained in wind tunnel tests with the aerofoil of FIG. 5 at a number of post-stall angles of attack, corresponding to flap deflection of +20° and Reynolds numbers of 0.3*10⁶.

FIGS. 11(a) and 11(b) illustrate hysteresis effect on lift coefficient variations with angle of attack obtained in wind tunnel tests with the aerofoil of FIG. 5 at flap deflection of 0° and Reynolds numbers of 0.3*10⁶.

FIGS. 12(a) and 12(b) illustrate hysteresis effect on lift coefficient variations with angle of attack obtained in wind tunnel tests with the aerofoil of FIG. 5 at flap deflection of +20° and Reynolds numbers of 0.3*10⁶.

FIG. 13 compares lift coefficient variations with angle of attack obtained in wind tunnel tests with the aerofoil of FIG. 5 at flap deflection of 0° and at flap deflection of +60°, at Reynolds numbers of 0.3*10⁶.

FIGS. 14(a) to 14(d) illustrate pressure distributions obtained in wind tunnel tests with the aerofoil of FIG. 5 at a number of post-stall angles of attack, corresponding to flap deflection of +60° and Reynolds numbers of 0.3* 10⁶.

FIG. 15 compares axial overlap variation with flap deflection angle, obtained with the aerofoil of FIG. 5 and obtained with baseline aerofoil SA-21.

FIG. 16 compares maximum lift coefficient variation with flap deflection angle, obtained with the aerofoil of FIG. 5 and obtained with baseline aerofoil SA-21.

FIG. 17(a) illustrates at Reynolds numbers of 0.5*10⁶, lift coefficient variations with angle of attack obtained with the aerofoil of FIG. 5 corresponding to flap deflections of 0°, +10°, +25°; FIG. 17(b) illustrates drag coefficient variations with flap angle obtained with the aerofoil of FIG. 5.

FIG. 18 illustrates a simplified estimation of glide angle with Lift/Drag ratio.

FIG. 19 illustrates the geometry and general dimensions of UAV AK-60.

FIGS. 20(a) and 20(b) illustrate flight test results of angle of attack variation with time, and stall velocity variation with time, respectively, obtained with UAV AK-60.

FIGS. 21(a) to 21(d) illustrate variations with time of angle or attack, glide angle, velocity, and rate of descent (ROD), respectively, at conditions of flap deflection angle +20°, elevator deflection angle −12° and engine idle, obtained with flight test of the UAV AK-60.

FIGS. 22(a) to 22(d) illustrate variations with time of angle or attack, glide angle, velocity, and rate of descent (ROD), respectively, at conditions of flap deflection angle +60°, elevator deflection angle −12° and engine idle, obtained with flight test of the UAV AK-60.

FIG. 23 schematically illustrates the wing lift coefficient variation with angle of attack for the UAV AK-60.

FIG. 24 illustrates variations of lift coefficient and drag coefficient with flap deflection angle obtained in two-dimensional wind tunnel testing of aerofoil SA-MS/17MLS.

FIG. 25 schematically illustrates various forces acting on an air vehicle during landing maneuver according to aspects of the presently disclosed subject matter; FIG. 25(a) schematically illustrates approximate balance of forces on the air vehicle of FIG. 25.

FIG. 26 illustrates the variation of the thrust lift force component, as a percentage of total thrust, with pitch angle θ of the thrust vector to the horizontal H.

FIG. 27 illustrates schematically various options for landing conditions with respect the wing lift coefficient variation with angle of attack for the UAV AK-60.

FIG. 28 illustrates controllability limits for the UAV AK-60, in terms of variation of maximum lift coefficient with angle of attack.

FIG. 29 illustrates spanload distributions of lift coefficient for an example of SA-MS wings.

FIG. 30 illustrates longitudinal stability of the UAV AK-60, in terms of variation of maximum lift coefficient with function of pitching moment.

NOMENCLATURE

-   -   C_(L) ^(1.5)/C_(D)—aircraft endurance factor α—angle of attack     -   C_(L)—aircraft lift coefficient δ_(ail)—aileron deflection     -   C_(d)—aerofoil drag coefficient δ—flap deflection     -   C_(l)—aerofoil lift coefficient C_(l)max—wing maximum lift     -   C_(l)max—aerofoil maximum lift AR—aspect ratio     -   C_(m)—aerofoil pitching moment CFD—computational fluid dynamics     -   C_(p)—pressure coefficient IAI—Israel Aircraft Industries     -   C_(D 0)—zero lift drag NLF—natural laminar flow     -   e—spanload efficiency MS—mild stall     -   Re—Reynolds number SA—slotted aerofoil     -   t/c—thickness ratio UAV—unmanned air vehicle     -   x/c—chord fraction W—Weight     -   TV—thrust vector (i.e., direction θ—pitch angle of the thrust of         thrust) vector to the horizontal     -   TH—forward thrust component H—horizontal or horizon     -   TL—thrust lift force component T—thrust

DETAILED DESCRIPTION

According to a first example of the presently disclosed subject matter, a powered aircraft, in particular a fixed wing powered aircraft is provided with high lift wings. For the purpose of example, and referring to FIG. 4, such an air vehicle or aircraft is described herein as a fixed-wing aircraft, generally designated with reference numeral 1, comprising a regular subsonic/transonic configuration, having a fuselage section 2, main wings 10 (only the starboard wing (also referred to herein as a “wing half”) is illustrated in this figure), tailplane 3, vertical stabilizers 4, and a propulsion system 5. However, the presently disclosed subject matter is also applicable, mutatis mutandis, to other types of aircraft, for example: subsonic/transonic aircraft having canards rather than a tailplane; general aviation aircraft, cruise missiles or other air delivered ordinance, and so on. Furthermore, while the presently disclosed subject matter finds particular application in UAV aircraft, the presently disclosed subject matter can also be applied to manned aircraft, mutatis mutandis, in particular to general aviation, subsonic transport, naval aviation, guided or other weapons, and so on, for example.

The air vehicle further comprises a control system comprising a controller (not shown) configured for operating the air vehicle during a landing maneuver as disclosed herein.

The propulsion system 5 is a controllable propulsion system configured for providing a thrust T. In some examples, this thrust can be produced directly from the engine as engine thrust—for example where the propulsion engine is a turbofan or turbojet. In other examples in which the propulsion engine drives one or more rotors, such as for example propellers or fans, the propulsion engine generates power that drives the rotor, rather than generate thrust directly, and the rotors each generate thrust aerodynamically as it turns in the air.

In particular, the propulsion system 5 is configured for providing a controllably changeable level of thrust T, ranging from a minimum thrust T_(MIN) at idle to a maximum thrust T_(MAX).

In the illustrated example of FIG. 4, and by way of non-limiting example, the propulsion system 5 is in the form of a forward mounted single puller propeller, though in alternative variations of the example, the propulsion system can include more than one puller propellers and/or one or more than one pusher propellers and/or one or more of: turbojets, propfans, turboprop, turbofans, and so on, mounted on the wings and/or fuselage or elsewhere with respect to the air vehicle. Herein the term “propeller” herein also refers to unducted fans, ducted fans, and other rotors that are configured for providing an aerodynamic propulsive thrust when driven by the propulsion engine.

In this example, the direction of thrust T for forward flight generated by the propulsion system 5, i.e., the thrust vector TV, is fixed with respect to the air vehicle, and is generally correlated to the longitudinal axis LA of the air vehicle, typically the thrust vector TV being at an angle λ to the longitudinal axis LA (in pitch), wherein angle λ can be zero. In this example, the direction of thrust T is parallel to the longitudinal axis LA and passes through the center of gravity, or the direction of thrust T can be inclined by angle λ within up to ±5° to the longitudinal axis LA in pitch, for example.

In the illustrated example of FIG. 4, and by way of non-limiting example, the wing has a substantially trapezoidal plan shape, the leading edge 52 of the wing 10 being substantially rectilinear and having a substantially zero sweep angle, and the wing having a taper of between about 0.6 to about 1.0, between the root 24 and the tip 22. The trailing edge 54 of the wing 10 is also substantially rectilinear and comprises a substantially negative sweep angle. In other variations of this example, the wing 10 can have a different plan form, for example: swept-back or swept forward, and/or with a different taper ratio (along the full wing, or different taper ratios for the inboard section and the outboard section), for example a zero taper ratio along the span of the wing 10; and/or having a different plan form, including curved leading edges and/or trailing edges such as an elliptical form, for example; and/or the inboard section and/or the outboard section can have a positive, negative or zero dihedral angle; and so on.

According to an aspect of the presently disclosed subject matter, the wing 10 is configured for providing high lift mild stall characteristics. This feature enables the air vehicle 1 to land at low speeds, including a range of post-stall speeds at high angles of attack, exploiting the relatively high lift coefficient still available at post-stall.

Referring also to FIG. 5, the wing 10 in this example is based on (i.e., the wing 10 comprises a plurality of aerofoil sections at least a portion of which each correspond to) a two-element mild stall slotted aerofoil 300, also interchangeably referred to herein as SA-MS aerofoil or SA-MS aerofoil 300, in particular exhibiting high lift mild stall characteristics. Aerofoil 300 comprises a first or primary element 32 (also referred to interchangeably herein as the “main body of the aerofoil”, or as the “main body”), and a pivotable second or secondary element 34 (also referred to interchangeably herein as the “flap” of the aerofoil, or as the flap).

The primary element 32 comprises the leading edge 38 of the aerofoil, which coincides with the leading edge 52 of the wing 10, and major portions 31 a, 33 a, of the suction surface 31 and pressure surface 33 thereof, respectively, and a trailing end 38 a, also referred to interchangeably herein as the trailing edge of the primary element 32. The secondary element 34 comprises the trailing edge 39 of the aerofoil, which coincides with the trailing edge 54 of the wing 10, and minor portion 31 b, 33 b, of the suction surface 31 and pressure surface 33 thereof, respectively. A slot 55 separates the leading end 35 of the secondary element 34 (also referred to interchangeably herein as the leading edge of the secondary element 34) from the trailing end 38 a of the primary element 32. For example, the slot 55 can have an axial width w at least 2% of the aerofoil chord C in a non-deflected position of the flap element.

Reference line RL is a datum line conventionally defined with respect to the aerofoil 300, and the angle of attack α of the aerofoil 300, and deflection angle δ of the secondary element 34, are defined with respect to this datum line. The deflection angle δ is also referred to interchangeably herein as flap deflection angle δ or as flap angle δ.

The precise form of the slot 55 can generally vary with deflection angle δ of the secondary element 34, and thus generally depends on the particular mode of operation of the wing 10, as illustrated, for example, in FIGS. 6(a) to 6(e) which refer to cruise/loiter mode (deflection angle δ=0°), take-off/landing mode (deflection angle δ=+25°), aileron mode (deflection angle δ=±25°), maximum speed decambering mode (negative flap deflection, deflection angle δ=−15°), and airbrake mode (deflection angle δ=+60°), respectively.

The hinge point 59 of the secondary element 34 is fixed at an outwardly displaced position with respect to the lower (pressure) surface of the secondary element 34. The airflow through slot 55 provides enhanced efficiency and linearity of the aerodynamic characteristics of the aerofoil as compared to a similar non-slotted aerofoil. A suitable actuation mechanism (not shown) is provided for actuating the secondary element 34 to adopt the range of deflection angles desired, including airbrake mode deflection angles.

In this example, the secondary element 34 is formed as a slotted flap segment, having a slotted flap construction, comprising sub-elements 34 a and 34 b, rigidly connected to one another and separated by minor slot 37, such that the secondary element 34 rotates about hinge point 59 as a rigid body. The slotted flap construction is configured for facilitating providing smooth lift characteristics at post-stall angles of attack at intermediate positive flap deflections with deflection angles δ of about 15° to about 25°. In alternative variations of this example, the secondary element 34 is formed as a single continuous flap segment, without the minor slot.

Referring to FIGS. 7(a) to 7(c), the secondary element 34 is spatially positioned with respect to the primary element 32 to provide a characteristic relationship between the longitudinal overlap M (between primary element 32 and the secondary element 34), and the deflection angle 6 of the secondary element 34 with respect to the primary element 32. According to this characteristic relationship, the longitudinal overlap M is unconventionally chosen to maximize the maximum lift coefficient that is obtained at deflection angles δ that are significantly larger than the deflection angle δ associated with maximum lift of the aerofoil, and in particular at deflection angles δ associated with airbrake mode of the aerofoil.

In airbrake mode, the secondary element 34 is deflected by a respective flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag. Thus, in airbrake mode, the flap angle δ is large, and generates significantly more drag than at flap angles associated with maximum lift coefficient of the aerofoil in which flow on the flap segment is attached or almost attached (while concurrently the maximum lift coefficient can remain almost constant). In particular, and as illustrated in FIG. 17(b), for example, in airbrake mode the flap angle δ can be more than 40°, or more than 50°, or more than 55°, or 60° or more than 60°. In each case, the respective drag coefficient is at least 150% of the value at flap angle δ of 30°, and the respective drag coefficient at these airbrake mode flap deflection angles δ can be greater than 0.15, or greater than 0.2, or greater than 0.3, for example. In particular, and as illustrated in FIGS. 14(a) to 14(d), for example, in airbrake mode the flow over the secondary element is fully detached.

The longitudinal overlap M thus chosen retains a high maximum lift for the aerofoil 300. Furthermore, and as will become clearer below, and the longitudinal overlap M thus chosen also allows the aerofoil 300 to produce mild stall characteristics. In turn, this allows for the integration of the secondary element 34 as an airbrake with a mild stall wing 10 and for flight of the aircraft 1 at post stall angles of attack, thereby retaining the plateau of high lift coefficient (associated with mild stall) with the secondary element 34 deployed as an airbrake.

Thus, and referring to FIGS. 7(b) and 7(c), at maximum conventional flap deflections associated with maximizing lift, with a deflection angle δ of about 25° for example, the longitudinal overlap M is about +4% of the chord, whereas at a deflection angle δ of about 60°, for example, associated with airbrake mode, the longitudinal overlap M is about −0.5% of the chord. These values of +4% and −0.5% for the longitudinal overlap M contrast with the corresponding longitudinal overlap values of +2% and −3.5%, respectively, obtained with the conventional aerofoil arrangement of FIGS. 2(a) and 2(b).

Thus, according to the aforesaid characteristic relationship, the longitudinal overlap M is unconventionally chosen to be larger, for example 1% or 2% or 3% or more than 3% of the chord larger, at any given flap angle, than would be the case if the longitudinal overlap M were conventionally chosen. Thus, for example, the longitudinal overlap M can be optimized to be within the range +2% to −0.5% (or to be within the range +1% to −0.5%) at deflection angles δ wherein the flap operates as an air brake—for example at airbrake deflection angle δ of 45° or more, or at airbrake deflection angle δ of 50° or more, or at airbrake deflection angle δ of 55° or more, or at airbrake deflection angle δ of 60° or more, or at airbrake deflection angle δ of 65° or more, or at airbrake deflection angle δ of 70° or more, or at airbrake deflection angle δ of 75° or more, or at airbrake deflection angle δ of 80° or more.

The numerical value for the flap angle δ and the longitudinal overlap M can be chosen to provide the desired level of drag (see FIG. 17(b)) and desired maximum lift coefficient and post stall lift characteristics.

In this example, the gap N between primary element 32 and the secondary element 34 can be set in a conventional manner, for example as set forth in FIG. 1, and for example kept at between about 1% and about 2% of the chord.

In this example, the location of the hinge point 59 is chosen to provide the desired longitudinal overlap M (and optionally also the desired gap N) as a function of the flap angle δ, and further, minimizes hinge moments, optimizes flap stability (providing the flap neutral point downstream of the hinge point 59), and complies with the conventional requirements of flap actuator failure case.

In alternative variations of this example, the secondary element 34 can comprise control surfaces such as ailerons, flaps and so on, that are pivotable with respect to the secondary element 34.

Referring again to FIG. 4, the wing half 10 can optionally comprise a number of span-wise wing sections, for example serially adjoining sections 10 a, 10 b, 10 c and 10 d, between the tip 22 and root 24, in which one or more wing section the secondary element 34 is configured to perform as an air brake, while in one or more other wing section the secondary element 34 can be configured as a rigid non-movable element, or to operate within a narrower flap angle range, for example up to 30° only, thereby not operating as an airbrake, for example. For example, section 10 a is configured with an aileron. For maximizing the drag effect of the airbrake, the span of the respective one or more sections 10 a, 10 b, 10 c and 10 d that include the airbrake can be maximized.

By way of non-limiting example, and referring again to FIGS. 5 and 6(a) to 6(e), such an aerofoil design for aerofoil 300 can include any suitable thickness-to-chord ratio (t/c)_(max), camber distribution, and thickness distribution. Optionally, the leading edge 38 of the aerofoil 300 can also be relatively blunt or rounded, having a low curvature leading edge radius, and aft portion of the aerofoil can also be cambered.

In this example, the aerofoil 300 further comprises a highly cambered aft portion of primary element 32, this aft portion being referred to herein as a mild-stall ramp section, or MS-ramp 154. The profile of the MS ramp 154 is shaped for a particular design Reynolds number, such as to produce a slowly creeping trailing edge separation at post-stall angles of attack. The MS ramp 154 is configured for initiating mildly separated flow at stall angles of attack, for example at about 10°. The separated region on the MS-ramp 154 at post stall angles of attack, for example at 15°, continues to grow slowly, but does not induce full separation over the primary element 32 of the two-element aerofoil 300, and enables a relatively high level of lift to be maintained at the post-stall angles of attack.

Without being bound to theory, inventors consider that when the secondary element 34 is deflected at flap angle δ corresponding to airbrake mode, the flow over the secondary element 34 is fully separated at all angles of attack, at least in the range of between 16° and 24°, and is the source of high parasitic drag. At the same time, there is a slow upstream progress of the separation region of the flow over the major portions 31 a of the suction surface 31 (of primary element 32) with increasing angle of attack, which accounts for the high lift mild stall characteristics of the aerofoil 300 even at airbrake mode.

Controllable development of the separated region on the MS-ramp 154, together with rounded pressure distributions at the leading edge 38, which can be obtained with a relatively blunt or rounded leading edge, having a low curvature leading edge radius, provide mild stall characteristics at the level of lift that is generally generated with high-lift, two-element aerofoils.

The upper suction surface 31 of the SA-MS aerofoil section, in particular the said MS ramp 154, includes portions of the downstream suction surface having a local curvature (1/r_(local)) that can increase, and/or that can remain substantially constant, i.e., where the local curvature does not substantially decrease, along the chord towards the trailing portion 38 a of the primary element 32, beginning at an intermediate portion of the suction surface 31 a thereof. The MS-ramp 154 is downstream of an upstream portion 152 of suction surface 31 a, in which the local curvature is reducing along the chord length. The rate of change of curvature along the suction surface 31 from the leading edge 38 to trailing portion 38 a can be positive in some variations of this example, or zero in other variations of this example.

Optionally, the point 153 on the suction surface 31 a along the chord at which the curvature stops decreasing, i.e., the transition between portions 152 and 154, can be located at any suitable location. For example, point 153 can be at any point between 15% and 25%, or at 30%, or at 40%, or at 45%, or at 50%, or at any point between about 30% and about 50%, or greater than 50% of the local chord (i.e., the chord of the primary element 32). Further optionally, the local curvature can be constant or increasing up to about 80%, 90%, 95% or more of the local chord.

Aerodynamically, the aerofoil 300 maintains a stall plateau for angle of attack range of at least 7° where the lift coefficient C_(l) is within up to about 5% of maximum C_(l).

According to some aspects of the presently disclosed subject matter, the MS ramp 154 can have a geometry that is dependent on the design Reynolds number for the aerofoil. For example, and without being bound to theory, inventors consider that the higher the design Reynolds number, the larger the local curvature of the MS-ramp 154 and the larger the longitudinal extent of the MS-ramp 154 over the major portions 31 a of the suction surface 31 (of primary element 32), and conversely, the lower the design Reynolds number, the lower the local curvature of the MS ramp and the lower the longitudinal extent of the MS-ramp 154 over the major portions 31 a of the suction surface 31 (of primary element 32). It is to be noted that the larger curvature MS-ramp that can be provided for a high design Reynolds number, also results in mild stall when the flow conditions are consistent with low Reynolds number, but at an additional drag penalty. By way of example, the design Reynolds number for a two element aerofoil can be between about 0.3*10⁶ to about 2.0*10⁶.

By way of non-limiting example, the aerofoil 300 comprises an 17% thickness ratio, and has Re_(design)=0.4*10⁶, and a particular geometry of such an aerofoil, corresponding to the geometry of the aerofoil 300 illustrated in FIG. 5, is particularly denoted herein as aerofoil SA-MS/17MLS.

Wind tunnel results obtained with a two-dimensional model of a zero-swept wing section, having a uniform cross-section along the span corresponding to aerofoil SA-MS/17MLS (corresponding to aerofoil 300), are shown in FIGS. 8(a) to 17(b).

Lift curves obtained for the aerofoil SA-MS/17MLS, at nominal flap setting (δ_(flap)=0°) and for conventional positive flap deflections of δ_(flap)=10° and of δ_(flap)=20° are shown in FIGS. 8(a) and 8(b), for Reynolds numbers of 0.3*10⁶ and 0.5*10⁶ respectively. These results indicate an extended range of plateau lift coefficients at post-stall angles of attack, at Reynolds numbers of 0.3*10⁶ and 0.5*10⁶. There is also a very gradual loss of lift at high post-stall angles of attack, as the flap deflections increase from δ=0° to δ=20°.

FIGS. 9(a) to 9(d) and FIGS. 10(a) to 10(d) show pressure distributions over the aerofoil SA-MS/17MLS (corresponding to aerofoil 300), at various post-stall angles of attack, and indicate a slow development of separation region on the respective MS-ramp of the aerofoil for flap deflections δ=0 and δ=+20°. Without being bound to theory, it is considered that rounded pressure distributions at the forward portion of the aerofoil prevent premature development of suction peak and delay a fast progress of separation at higher angles of attack; eventually, development of suction peak at high angles of attack accelerates the separation process at the aft portion of the main body, producing a very gradual loss of lift with increasing angles of attack.

Referring to FIGS. 11(a) to 11(b), and FIGS. 12(a) to 12(b), aerofoil SA-MS/17MLS (corresponding to aerofoil 300) does not show signs of hysteresis phenomena up to very high post-stall angles of attack, greater than about 32°. These results were obtained from a series of specially performed hysteresis test for aerofoil SA-MS/17MLS at zero flap deflection (δ=0°) and for flap deflection of δ=+20°. At angles of attack of up to at least 29°, the lift coefficient obtained with aerofoil SA-MS/17MLS (corresponding to aerofoil 300) is not sensitive to the direction in which the angle of attack is changing. In other words, increasing the angle of attack to about 30°, and then decreasing the angle of attack provides the same variation of lift coefficients with angle of attack. However, at an angle of attack of about 34°, there is a change, and lower lift coefficients are obtained at angles of attack between 34° and about 12° as the angle of attack is reduced, as compared with the corresponding lift coefficients obtained at similar angles of attack but as the angle of attack is increased. The hysteresis behavior thus appears to start at the relatively large angle of attack of about 34°, and this allows safe operation of the air vehicle at high post-stall angles of attack up to a little under 34°, for example up to 32° or 33°. In practical terms, for UAV's or other aircraft, this corresponds to effectively eliminating hysteresis phenomena up to relatively high post stall angles of attack.

Lift curve obtained with aerofoil SA-MS/17MLS (corresponding to aerofoil 300) in airbrake mode, with a flap deflection of δ=+60° is compared with the corresponding lift curve at zero flap deflection (δ=0°) in FIG. 13. These results clearly show acceptable mild-stall characteristics at post-stall angles of attack at the large flap deflections associated with airbrake mode.

FIGS. 14(a) to 14(d) illustrate pressure distributions obtained over aerofoil SA-MS/17MLS at various post-stall angles of attack, with the secondary element deployed in airbrake mode at flap deflection of δ=+60°. These figures show development of separated region over the major portions 31 a of the suction surface 31 (of primary element 32) at post stall angles of attack, while the flow over the secondary element 34 is fully separated at the high flap deflection angles of airbrake mode. Comparison of FIGS. 14(a) to 14(d) with FIGS. 10(a) to 10(d) shows that there are no significant losses of maximum lift relative to the case of flap deflection at δ=+20° for the same aerofoil, allowing safe flight of the aircraft at post-stall angles of attack with deployed airbrake for aerofoil SA-MS/17MLS (corresponding to aerofoil 300).

The above results indicate that the design of aerofoil SA-MS/17-MLS (corresponding to aerofoil 300), including the airbrake mode thereof, is suitable for integration with high-lift, mild-stall wing, allowing safe operation of an air vehicle, such as for example a UAV, at post-stall angles of attack and as will become clearer below also allows for providing relatively steep glides angles at approach to landing at reduced airspeeds.

The geometry of aerofoil SA-MS/17MLS (corresponding to aerofoil 300) can be compared with the geometry of a conventional baseline slotted aerofoil of the same thickness ratio, and denoted herein as aerofoil SA-21, illustrated in FIGS. 2(a) and 2(b), and while otherwise being similar to aerofoil SA-MS/17MLS, the baseline aerofoil SA-21 does not comprise an MS ramp (nor is configured to provide mild stall characteristics), and includes a conventional longitudinal overlap and a conventional gap, based on conventional optimization characterized by FIG. 1. Thus, conventional flap deflection angle δ of +25° associated with maximum lift, the axial overlap is about 2% of the aerofoil chord, while at large flap deflections associated with airbrake mode, for example deflection angle δ of +60°, the axial overlap is highly negative—about −3.5%. The variation of maximum lift coefficient with flap angle for the baseline aerofoil SA-21 is illustrated in FIG. 3.

FIG. 15 shows the geometrical variation of longitudinal overlap with deflection angle δ, for a range of deflection angle δ from 0° to 80°, obtained for aerofoil SA-MS/17MLS (corresponding to aerofoil 300). While not shown, the longitudinal overlap provided by aerofoil SA-MS/17MLS is greater than that of baseline aerofoil SA-21 by about 2% of the chord, for a range of deflection angle δ from 0° to 40°, thereafter this difference increasing to with deflection angle δ to about 3% of the chord at deflection angle δ of 80°.

FIG. 16 compares the variation of maximum lift coefficient C_(lmax) with deflection angle δ, for a range of deflection angle δ from 0° to 80°, obtained for aerofoil SA-MS/17MLS (corresponding to aerofoil 300). This figure further compares this variation, with the corresponding variation of maximum lift coefficient C_(lmax) with deflection angle δ, obtained for baseline aerofoil SA-21.

Thus, while the conventional design baseline aerofoil SA-21 shows an abrupt loss of maximum lift coefficient for flap deflections above δ_(flap)˜30°, the aerofoil SA-MS/17MLS (corresponding to aerofoil 300) provides a generally constant maximum lift variation (within a band of lift coefficient C_(l) between about 2.3 and about 2.55, i.e., a variation of about ±5% or ±6% of the mean value of maximum lift coefficient) with deflection angle δ, even at large flap deflections of 60° or 80°.

The inventors consider that the concept of providing longitudinal overlap according to the presently disclosed subject matter is particularly suitable for integration with high-lift, mild-stall wings, which are configured for providing mild stall characteristics i.e., a plateau of lift coefficients at extended range of post-stall angles of attack.

Without being bound to theory, the inventors further consider that this feature of generally constant maximum lift variation with flap deflection angle, results from providing the aforesaid longitudinal overlap optimized at the large deflection angles associated with airbrake mode, which is a radical departure from conventional design principles that require small positive overlap at δ_(flap)=25° to 30° for the design of single slotted flap.

It is to be noted that for conventional airbrake designs, for example the baseline aerofoil of FIGS. 2(a) and 2(b), the loss of maximum lift when the airbrake is deployed is of minor importance. This is because, conventionally, there is also imposed a speed safety margin constraint on the operation of the air vehicle, in which the speed of the air vehicle has to exceed the stall speed by at least 20%. i.e., V_(flight)>1.2 V_(stall) (and a corresponding lift coefficient safety margin constraint on the operation of the air vehicle, in which the lift coefficient of the wings of the air vehicle has to remain below the maximum lift coefficient by a factor of 1/1.44 (or must remain at under 70% of the maximum lift coefficient), i.e., C_(L flight)<C_(Lmax)/1.44. However, according to aspects of the presently disclosed subject matter, the high-lift mild stall characteristics of the aerofoils allow elimination of this speed safety margin and support safe flight of the aircraft at post-stall angles of attack maintaining high levels of lift.

It is to be noted that in alternative variations of this example, the respective aerofoil is not configured for providing high lift mild stall characteristics or for providing mild stall characteristics, though the respective aerofoil is otherwise configured as a two element slotted aerofoil, mutatis mutandis. The respective secondary element of such an aerofoil, pivotable about a fixed hinge joint and having a longitudinal overlap and gap as disclosed herein for aerofoil 300 will also enable the respective secondary element to operate in airbrake mode, and without being bound to theory inventors consider that such a configuration can help in moderating stall characteristics at high flap deflection angles corresponding to airbrake mode.

Experimental lift curves with angle of attack, obtained with aerofoil SA-MS/17MLS (corresponding to aerofoil 300) at flap deflections of δ_(flap)=0°, δ_(flap)=+10° and δ_(flap)=+25°, are shown in FIG. 17(a), illustrating for δ_(flap)=+25° the achieved level of smooth plateau of lift coefficients of about 2.6 at extended range of post-stall angles of attack of between at least 4° and about 18°.

Thus, the combination of providing longitudinal overlap according to some aspects of the presently disclosed subject matter with high-lift, mild-stall two-element aerofoils retains mild-stall characteristics at extended range of post-stall angles of attack and at all flap deflections at least up to δ_(flap)=+80°. At the same time, the secondary element of the aerofoil provides a very high level of parasitic drag when in airbrake mode, as illustrated in FIG. 17(b), allowing approach to landing of an aircraft, for example a UAV, at reduced airspeeds at post-stall angles of attack with increased glide angles. This is not generally possible for high-lift, mild-stall wing with a conventional airbrake design because of the loss of lift at high deflection angles, and the resulting inability to provide safe flight at post-stall angles of attack.

According to another aspect of the presently disclosed subject matter, the slotted aerofoil including longitudinal overlap according to the presently disclosed subject matter provides a particularly efficient airbrake for operation at approach to landing.

A simplified definition of glide angle y is provided by the following equation:

γ=1/(L/D)   (A)

(i.e., glide angle γ=D/L)

. . . where L is the lift force and D is the drag force acting on the air vehicle at approach to landing. It is to be noted that, conventionally, glide angles below the horizon are considered negative, while glide angles above the horizon, i.e., climb angles, are considered positive.

FIG. 18 illustrates the variation of glide angle γ with lift-to-drag ratio (L/D). Thus, high (negative) values of glide angle γ are obtained for low values of the lift-to-drag ratio (L/D). However, large lift is usually desired at landing in order to provide a low landing speed, which would result in low (negative) values of glide angle γ.

For example, high (negative) values of glide angle γ can be provided in airbrake mode, for example in the range of between −3° to about −15°, for example in the range of between −10° to about −15°, for example −10°, or −11° or −12° or −13° or −14° or −15° or numerically less than −15°, with corresponding flap angles. For example glide angle γ can be provided in the range −10° to −15°, for example glide angle γ can be −10°, or −11° or −12° or −13° or −14° or −15°, and correspond to a flap deflection angle δ of about +60°, for example, in airbrake mode.

Maximizing the modulus of glide angle γ can minimize exposure time of the air vehicle during landing. This can be of particular use in maritime applications, for example, wherein exposure to gusts etc. can be minimized when landing on a sea platform, such as an aircraft carrier, for example, particularly on rough seas. The ability to provide large (negative) glide angles γ can also be useful where tall obstacles are located in the vicinity or perimeter of the landing site for example.

The capability to controllably provide a wide range of glide angles, particularly during the actual landing, also allows for precision of touch down point of the air vehicle.

According to an aspect of the presently disclosed subject matter, the aerofoil 300 is configured to generate large values of drag concurrently with high lift (which provides low landing velocity) in airbrake mode—the high drag thus compensates for the increased lift at landing, to maintain low values of the lift-to-drag ratio (L/D), and thus provide high absolute values of (negative) glide angle γ.

According to some aspects of the presently disclosed subject matter, the pivotable second or secondary element 34 capable of operating in the aforesaid airbrake mode, is referred to as high lift-high drag airbrake.

According to an aspect of the presently disclosed subject matter, the air vehicle 1, during a landing maneuver, adopts a relatively high angle of attack α, which concurrently provides a thrust vector TV having a significant vertical thrust component TL that contributes to the lift force on the air vehicle 1. The landing maneuver can include approach to landing and/or landing of the air vehicle 1. Referring to FIG. 25, according to an aspect of the presently disclosed subject matter the air vehicle 1 is following a glide path indicated at FP during a landing maneuver, the glide path being at glide angle γ to the horizontal H (i.e., to the Earth horizon, and below the Earth horizon), while the air vehicle 1 (for example, the longitudinal axis LA of the air vehicle 1) is at an angle of attack α.

In other words, the thrust vector TV is at a pitch angle θ to the horizontal H, wherein:

α=θ−γ

. . . and wherein as already mentioned glide angles γ below the horizon are defined as having negative values.

Thus, the aerodynamic lift force L generated by the wings is orthogonal to the flight path FP, the drag force generated by the air vehicle is parallel to the flight path FP, while the weight of the air vehicle acts orthogonal to the horizon H. On the other hand the thrust vector TV of the thrust T is at angle θ to the horizontal H.

It is to be noted that the thrust vector TV is at an angle λ to the longitudinal axis LA (in pitch), and thus the longitudinal axis LA is at angle θ′ to the horizontal H, wherein angle θ=θ′+λ. In the above example, the thrust vector TV is aligned with the longitudinal axis LA, thus angle λ is zero, and thus angle θ′ equals angle θ. In alternative variations of this example, angle λ can be positive or negative, and thus the longitudinal axis LA can be at a greater or less pitch angle θ′ as compared with the angle θ of the thrust vector TV to the horizontal.

Referring to FIG. 25(a), the thrust vector TV resolves the thrust T into a thrust lift force component TL orthogonal to the horizontal H, and a forward thrust component TH parallel to the horizontal H.

The thrust lift force component TL and the forward thrust component TH are related to the thrust T and the angle θ as follows:

TL=T*sin θ

TH=T*cos θ

The total lift force L_(TOTAL) that now balances the weight W of the air vehicle is approximately:

L_(TOTAL)˜L+T*sin θ

Or, more precisely:

L _(TOTAL=) L*cos γ+T*sin θ

FIG. 26 illustrates the variation of thrust lift force component TL (as a percentage of total thrust T) with pitch angle θ. Typically, glide angles γ can vary between about −4° and about −1.5° during landing, for example—see FIG. 18—and thus pitch angle θ can be very close to the angle of attack α. As can be readily appreciated, providing a pitch angle θ of 20° provides a thrust lift force component TL of about 34% of the thrust T generated by the propulsion system 5, while a pitch angle θ of 30° provides a thrust lift force component TL of about 50% of the thrust T. It is evident that according to this aspect of the presently disclosed subject matter, the thrust T of the propulsion system 5 can significantly contribute to the total lift force L_(TOTAL) acting on the air vehicle during landing.

According to an aspect of the presently disclosed subject matter, the thrust contribution to lift provided by the thrust lift force component TL by virtue of providing a high pitch angle θ, does not come as a detriment to the aerodynamic lift force L provided by the wings. According to this aspect of the presently disclosed subject matter, the pitch angle θ is chosen to correspond to the angle of attack α (of the air vehicle 1) in the post-stall regime of the mild stall wings, where the lift coefficient C_(L), post stall, remains high as angle of attack is increased post stall. Referring to FIG. 23, the lift coefficient in the post-stall regime remains high at about 2.5±0.1 for a range of angle of attack between 10° and 20°. Thus, operating the air vehicle 1 in this post-stall regime of angles of attack provides high lift coefficient, and also allows for a high thrust lift force component TL.

Furthermore, it is to be noted that the higher the absolute value or level of thrust T, the greater the contribution of thrust lift force component TL to the total lift, for a given pitch angle θ. However, it is also to be noted that as absolute value of thrust T is increased, the forward thrust component TH also concurrently increases.

At a constant forward speed, the forward thrust component TH approximately balances the drag D:

T*cos θ˜D

Or, more precisely:

T*cos θ=D*cos γ−L*sin γ

Conventionally, horizontal landings are carried out with little or no thrust, with the engine typically at idle setting, and thus relationship (A) is generally applicable (i.e., glide angle γ=D/L). However, if, for the sake of argument, the absolute value of thrust T is increased, this will conventionally result in an increase the forward velocity of the air vehicle 1, which is generally not desired at landing. In such a situation, a more precise definition of glide angle γ when there is significant thrust (T) is provided by the following equation:

γ=(D−T)/L   (B)

It is to be noted that for a highly efficient aerodynamic configurations for the air vehicle (i.e., having low drag characteristics, even at landing) the resultant glide angles will be numerically very low, requiring a very long landing strip to eventually touchdown. In such cases such excess thrust at landing can cause the air vehicle to essentially “float” above the runway under ground effect for long distances, and if the excess thrust cannot be eliminated airbrakes or spoilers need to be used to increase the drag D and thus numerically increase the glide angle. According to at least one aspect of the presently disclosed subject matter, the glide angle is numerically increased (in the negative sense) by vastly increasing the drag generated at landing, this being accomplished by operating the air vehicle in the aforesaid airbrake mode.

According to an aspect of the presently disclosed subject matter, high levels of absolute value of thrust T are maintained, providing a significant contribution of thrust lift force component TL to the lift of the air vehicle 1, while not increasing the speed of the air vehicle at landing. Rather, the airspeed of the air vehicle can be actually reduced. This is achieved, according to the aspect of the presently disclosed subject matter, by concurrently deploying (in at least one example) the air brakes at high flap deflection angles corresponding to airbrake mode, thereby providing high levels of drag while maintaining the lift coefficient relatively constant. In other words, according to this aspect of the presently disclosed subject matter the air brakes are used in airbrake mode to increase the drag, while not substantially affecting lift of the wings. In other words, operation of the engine at power levels higher than idle (at high post-stall angles of attack) is rendered possible in view of the compensating high levels of drag generated at airbrake mode. In at least some cases the air vehicle is configured such that the total drag exceeds the thrust generated during landing, the difference between the drag and the thrust defining the glide angles (see relationship (B) above) which are lower relative to conventional type of landing (at similar drag levels).

It is to be noted, by way of example, that since at an angle of attack a corresponding to pitch angle θ of 20°, 34% of the generated thrust is converted to lift force (see FIG. 26), it follows that at partial engine setting only 34% of actual thrust that is generated will contribute to the lift. Similarly at different angles of attack and different engine settings, mutatis mutandis.

It is further to be noted that the thrust contribution to lift, provided by various aspects of the presently disclosed subject matter, allows operation of the air vehicle at airspeeds (V_(flight)) that are below the stall speed of the air vehicle (V_(stall)), for example airspeeds (V_(flight)) of between 90% and 95% of the stall speed V_(stall), at high-post stall angles of attack. It is also to be noted that during landing, the weight of air vehicle is supported by reduced lift force (due to reduced air speed at landing) plus thrust contribution to lift (generated by the engine) TL, as illustrated in FIG. 25 and FIG. 25(a). It is therefore apparent that, rather than being constrained to operate the air vehicle at the conventional speed safety margin constraint (V_(flight)>1.2V_(stall)), the method of operation of the air vehicle at landing, according to at least some aspects of the presently disclosed subject matter, allows operation of the air vehicle at high post-stall angles of attack concurrently with airspeeds (V_(flight)) between 90% and 95% of V_(stall), when V_(stall) is an average airspeed corresponding to the post-stall plateau of lift coefficients corresponding to the mild stall characteristic of the wings of the air vehicle 1.

Thus, operation of the air brakes in airbrake mode to increase the drag enables the level of thrust T to be set relatively high (and much higher than idle, for example), thereby allowing for a significant contribution of thrust lift force component TL to the lift of the air vehicle 1, while the additional forward thrust component TH is offset by the increased drag generated by the air brakes. (It is to be noted that in at least some cases, the thrust contribution to lift can be quite moderate (for example with the engine operating at only partial thrust), but can still be significant as it enables a reduction of air vehicle kinetic energy, and furthermore this effect can be increased by the additional speed reduction that can be obtained due to high lift effect and elimination of speed safety margin.)

In this manner, the air vehicle can be decelerated and operated at relatively low airspeeds during landing, while at the same time generating relatively large lift force via the thrust lift force component TL. In fact it is possible to control the level of drag generated by the airbrakes and the level of thrust T so that in fact the forward thrust component TH is less than the drag, thereby resulting in a deceleration of the air vehicle 1. Furthermore, and as discussed above, the mild stall characteristic of the wings of the air vehicle 1 further allows the stall margin to be exploited, and allow the air vehicle 1 to operate at even slower speeds during the landing maneuver.

In turn, this allows for a greater (negative) glide angle γ to be used for the landing maneuver, allowing a shorter landing distance, facilitating avoidance of ground obstacles and/or providing accuracy of touch down location in ground effect. As discussed above, the relative large levels of drag that can be generated by the operation of the air brakes in airbrake mode (while not concurrently increasing lift) allows for such relatively large (negative) glide angle γ (see relationships (A) and (B) above).

According to aspects of the presently disclose subject matter, the level of thrust T during a landing maneuver can be any one of the following percentages of the nominal maximum thrust T_(MAX): 5%; 10%; 15%; 20%; 25%, 30%; 35%, 40%; 45%; 50%; 55%; 60%; 65%; 70%; 75%; 80%; 85%; 90%; 95%.

According to aspects of the presently disclose subject matter, the level of thrust T during a landing maneuver can be within any one of the following ranges of percentages of the nominal maximum thrust T_(MAX): 1% to 5% and/or 5% to 10% and/or 10% to 15% and/or 15% to 20% and/or 25% to 30% and/or 30% to 35% and/or 35% to 40% and/or 40% to 45% and/or 45% to 50% and/or 50% to 55% and/or 55% to 60% and/or 60% to 65% and/or 65% to 70% and/or 70% to 75% and/or 75% to 80% and/or 80% to 85% and/or 85% to 90% and/or 90% to 95 and/or 95% to 100%.

It is to be noted that in alternative variations of this example, for example where the air vehicle is not fitted with the above airbrakes, other drag inducing devices can be employed which selectively induce drag at landing while not substantially affecting the flow over the wings. Thus drag inducing devices do not significantly affect the wing aerodynamics including lift coefficient. Such drag inducing devices can include, for example, deployable airbrakes on the fuselage, deployable parachutes, and so on.

According to aspects of the presently disclosed subject matter, and referring to FIG. 27, various landing methods can be employed by the air vehicle 1, and four such methods, denoted herein as landing modes A, B, C and D, are discussed below.

Landing Mode A

In this landing mode, the air vehicle lands in a conventional manner, complying with the standard speed safety margin in which the velocity of the air vehicle is at least 20% greater than the stall speed (i.e., V_(flight)>1.2V_(stall)). At these conditions, represented at “A” in FIG. 27, the angle of attack of the wings is relatively low, as is the air vehicle coefficient of lift C_(L). Furthermore, while the high lift-high drag airbrake may provide improved glide angles, there is no significant advantage provided by the aforesaid high lift-high drag airbrake, i.e., the potential high drag and high lift characteristics of the airbrake are not generally realized under these conditions. Furthermore, landing in mode A is conducted with the propulsion system providing idle thrust; in any case, even if thrust is significantly increased from idle, there is no significant contribution of thrust to lift because of the very low angle of attack, and thus of the pitch angle θ. On the other hand, deployment of standard air brakes can provide acceptable glide angles in at least some case wherein the absence of such air brakes provides very shallow glide angles.

Landing Mode B

In this landing mode, the air vehicle lands in a non-conventional manner, eliminating the speed safety margin of V_(flight)>1.2V_(stall), and the propulsion system operates at idle thrust. In these conditions, represented at “B” in FIG. 27, the angle of attack of the wings is relatively high, and the air vehicle coefficient of lift C_(L) is at a maximum, at stall. Furthermore, the high lift-high drag airbrake provide improved glide angles, as well as increased parasitic drag, providing greater rate of descent. In landing mode B, operation of the airbrake in airbrake mode can ensure flight safety at stall conditions and provides improved glide angles as compared with landing mode A. On the other hand, thrust is maintained at idle, in view of the degradation of (negative) glide angle at approach to landing, minimizing any potential lift benefits of increasing engine thrust.

Landing Mode C

In this landing mode, represented at “C” in FIG. 27, the air vehicle lands in a non-conventional manner, eliminating the speed safety margin of V_(flight)>1.2V_(stall), and the propulsion system operates at idle thrust, in a similar manner to landing mode B. However, in landing mode C, the angle of attack is much greater than in landing mode B, but still within the angle of attack range in the plateau of lift coefficients, marked at P in FIG. 27. The increased angle of attack increases the effect of the high lift-high drag airbrake, providing increased parasitic drag, improved rate of descent as compared with landing mode B. The mild stall high lift nature of the wings allows operation of the wings for an extended range of angles of attack.

Once the speed safety margin is eliminated, and there exists a plateau of lift coefficients at an extended range of post-stall angles of attack (for example P in FIG. 27, landing mode C becomes in general preferable to landing mode B. Operation of the air vehicle at such angles of attack corresponding to landing mode B can be accomplished, for example, by providing and maintaining a corresponding negative elevator setting to the air vehicle.

Landing Mode D

In this landing mode, represented at “D” in FIG. 27, the air vehicle lands in a more non-conventional manner than at “C”, eliminating the speed safety margin of V_(flight)>1.2V_(stall), and the propulsion system operating at a thrust that is significantly above idle thrust. In addition, in landing mode D, the angle of attack is greater than in landing mode C, being at the higher end of the angle of attack range in the plateau of lift coefficients P. At these angles of attack, the thrust contribution to lift is significant, allowing for further reduction in airspeed.

It is to be noted that, in practice regarding Landing Modes C and D, a new reduced level of stall speed (V_(stall)) is established (as compared with landing mode A, for example), and at the reduced dynamic pressure corresponding new stall speed, the weight of the air vehicle is supported only partially by the lift force, the rest of the weight being supported by the thrust contribution to lift.

At least in cases in which the propulsion system of the air vehicle includes a propeller, the resulting slipstream effect can also reduce the airspeed further. In the slipstream effect, the propeller generates a higher dynamic pressure (i.e. higher than that due to the forward speed of the air vehicle) over the inboard portions of the wings and fuselage, providing additional lift and thus allowing for reducing airspeed further. At the same time, the high drag generated by the high lift-high drag airbrake, not only provides increased parasitic drag as compared with landing mode C, it also compensates for the forward thrust component of the increased thrust, thereby enabling reduced landing speed in spite of the thrust being higher than idle. It is to be noted that landing mode D provides the minimum possible landing speed at touchdown, as compared with landing modes A, B or C.

Thus, in landing mode D reduced airspeed (even below the stall airspeed, for example 90% to 95% of V_(stall)) can be achieved because of the thrust contribution to lift, while the reduced dynamic pressure corresponding to such reduced airspeeds the physical lift generated by the air vehicle is insufficient to support the weight of air vehicle, the deficiency in aerodynamic lift is made up by the thrust contribution to lift.

In landing mode D there is a decrease of (negative) glide angle as compared with landing mode C. In landing mode D, high levels of drag at high post-stall angles of attack and at airbrake mode compensate for the forward thrust resulting from operating the engine at thrust levels higher than idle, and as shown above the difference between the drag and forward thrust can be sufficient to provide minimum required glide angles (of between −3° and −4°, for example).

Experimental Results with Wing Technology Demonstrator

A UAV, designated AK-60, was developed by the present assignee as a fully instrumented Wing Technology Flight Demonstrator, with the specific purpose to evaluate in-flight high-lift, mild-stall wings at domain of post-stall angles of attack and specifically the option of landing at stall at reduced airspeeds with increased glide angles employing the feature of the airbrake concept according to the above aspects of the presently disclosed subject matter.

The UAV AK-60 has a conventional wing-body-tail arrangement that is shown in FIG. 19, which also shows various overall dimensions of this UAV. The main wings of the UAV AK-60 configuration were high-lift, mild-stall wings, comprising airfoil sections corresponding to airfoil SA-MS/17-MLS (see FIGS. 5 to 7(c)), which included the feature of the airbrake concept according to the above aspects of the presently disclosed subject matter.

Two technical objectives were specified for development of UAV AK-60. The first one was of a primary nature—demonstration of advantages of landing at stall at reduced airspeeds and with relatively steep glide angles via implementation of innovative design concepts (high-lift, mild-stall wing, the feature of the airbrake concept and thrust contribution to lift according to the above aspects of the presently disclosed subject matter). The second one was of supplementary nature—verification of flight safety and validation of acceptable flying qualities up to high post-stall angles of attack.

The primary objective of the demonstration was supported by the following elements:

-   -   large wing area/low wing loading for providing reduced airspeeds         at all stages of the flight;     -   implementation of high-lift, mild-stall SA-MS aerofoil-based         wing and elimination of speed safety margin         V_(flight)>1.2V_(stall);     -   the use of high-lift devices for producing high maximum lift         (actually maximum usable lift because of elimination of speed         safety margin);     -   implementation of the feature of the airbrake concept according         to the above aspects of the presently disclosed subject matter         for providing increased glide angles at approach to landing         without penalizing lift and maximum lift characteristics at         post-stall angles of attack and retaining reduced airspeeds with         deployed airbrake;     -   exploiting thrust contribution to lift at high post-stall angles         of attack for a further reduction of airspeed at approach to         landing, the forward thrust thereby generated being         counterbalanced by airbrake drag for retaining acceptable glide         angles.

The concept of landing at stall was evaluated in a number of flight tests of UAV AK-60 with a slotted mild stall wing and including the feature of the airbrake concept according to the above aspects of the presently disclosed subject matter. Most of flight testing was performed with a slotted mild stall wing based on airfoil SA-MS/17MLS, incorporating the feature of the airbrake concept according to the above aspects of the presently disclosed subject matter. A limited number of flight tests were performed with a slotted mild stall wing that was based on a single element high lift airfoil (as disclosed in U.S. Pat. No. 7,992,827, “Wings for Aircraft”, Ref. 6).

The evaluation yielded the following results:

-   -   Flight at post-stall angle of attack (α˜15°)         -   Characteristics of the flight at post-stall angles of attack             of about 15° are shown in FIG. 20(a) and FIG. 20(b). As             anticipated by the inventors, the average airspeed at             post-stall was close to stalling speed of V_(stall)˜30 kts.     -   Landing at stall         -   Approach to landing at post-stall angles of attack is shown             in FIGS. 21(a) to 21(d), for flap deflection angle             δ_(flap)=+20°, and FIGS. 22(a) to 22(d), for flap deflection             angle corresponding to airbrake mode, δ_(airbrake)=+60°. For             both cases the approach to landing was performed at a             respective angle of attack corresponding to the middle of             post stall plateau range of lift coefficients (because of             the nose-down tendency of the air vehicle at high angle of             attack), with an elevator deflection angle δ_(el) of −12°,             and with the engine setting of the air vehicle at idle.             Comparison of FIG. 21(b) with FIG. 22(b) clearly show a 50%             numerical increase in the glide angle γ, which increased (in             a negative angular direction) from −8° at flap deflection             angle of +20°, to −12° at flap deflection angle of +60°.             Similarly, comparison of FIG. 21(d) with FIG. 22(d) clearly             show an 18% increase in the rate of descent (ROD), which             increased from 490 ft/sec at flap deflection angle of +20°,             to 580 ft/sec at flap deflection angle of +60°. These             improvements in glide angle and rate of descent were             achieved without affecting the approach airspeed of the air             vehicle, and inventors suggest that this indicates that the             incorporation of the feature of the airbrake concept             according to the above aspects of the presently disclosed             subject matter to high lift mild stall wings does not             distort the variation of lift coefficient with angle of             attack at the post-stall plateau of lift coefficients of             mild stall wings, and moreover retains the maximum lift             coefficient of the baseline wing configuration at             conventional flap deflection angles of about +20°. Thus, the             feature of the airbrake concept according to the above             aspects of the presently disclosed subject matter did not             adversely affect the airspeed of the air vehicle, and             improved significantly the glide angles and rate of descent             of the air vehicle.

A number of supplementary design objectives were provided for flight safety of the air vehicle at post-stall angles of attack. These objectives were supported by the following design features:

-   -   the air vehicle flying at post-stall angles of attack should be         fully controllable in all stability planes. Theoretical stall         simulations were performed, and this helped to establish         controllability limits on conservative basis that are shown in         FIG. 28. It was predicted that up to α=20°, no degradation of         longitudinal and roll control are to be expected, ensuring         flight safety at post-stall angles of attack. The actual flight         testing was performed up to α=25°, indicating fully controllable         flight at high post-stall angles of attack     -   improved controllability in the roll plane at post-stall angles         of attack was achieved by combination of taper ratio and twist         that produced a gradual decrease of the local loading across the         span of the wing (see FIG. 29). The wing was designed to start         to stall at its inboard stations, with outboard sections of the         wing still being in the linear range of lift coefficients         keeping substantial margin from local maximum lift limit and         ensuring acceptable aileron efficiency up to high post-stall         angles of attack     -   pitching moment curves of UAV AK-60 (see FIG. 30) provide         inherent passive self-recovery of air vehicle in the         longitudinal plane. Gradual stall of the inboard wing portion         reduce the downwash on horizontal tail and cause the increase of         tail loading (see FIG. 28). This produces a strong pitch-down         tendency at post-stall angles of attack, preventing         uncontrollable increase of angle of attack. This type of         configuration may be flown at post-stall only when locked at         some post-stall angle of attack by negative setting of elevator

The thrust contribution to lift concept, particularly together with airbrake concept, according to at least some of the above aspects of the presently disclosed subject matter supports a corresponding innovative approach of UAV landing, in which the UAV lands with reduced airspeeds and steep glide angles.

The thrust contribution to lift concept, particularly together with airbrake concept, according to at least some of the above aspects of the presently disclosed subject matter allows for providing a method of operation of air vehicles, such as for example tactical UAVs, at approach to landing and landing, and can integrate one or more of the following features (see FIG. 23):

-   -   two-element, high-lift mild-stall wing that provides a plateau         of high-lift coefficients up to high post-stall angles of attack         and allows elimination of standard speed safety margin         V_(flight)>1.2V_(stall). This allows the approach airspeed to be         reduced below the above standard speed margin and provide         greater lift. This supports the concept of landing at stall,         benefiting from maximum usable lift for reduction of airspeed at         approach to landing to stall speeds, and providing high level of         drag for enhancing glide angles at final stages of approach to         landing (high level of induced drag at high lift coefficients         and increased parasite drag at post-stall angles of attack due         to flow separation).     -   the airbrake concept according to at least some of the above         aspects of the presently disclosed subject matter that generates         very high level of drag without penalizing lift characteristics         of mild-stall wing at post-stall angles of attack (see FIG. 24).         This allows increased glide angles at approach to landing         without compromising on UAV airspeed.

Thus, the combination of features of high-lift, mild-stall wing, the airbrake concept, and thrust contribution to lift, according to at least some of the above aspects of the presently disclosed subject matter) provides novel methods of operation of air vehicles, in particular UAV, by operating the air vehicle at very high post-stall angles of attack and maintaining engine power such as to generate thrust much higher than at engine idle conditions, thereby automatically providing a thrust contribution to lift. Features of at least some such methods of operation can include landing at reduced airspeeds and relatively steep glide angles, which in turn helps to achieve short landing distance and provides the capability to avoid ground obstacles at the final stages of approach to landing.

On the other hand it is to be noted that the airbrake concept according to at least some other of the above aspects of the presently disclosed subject matter can be integrated in other types of slotted wings which are not necessarily mild stall or high lift mild stall wings. In such cases, this allows for providing a method of operation of air vehicles, such as for example tactical UAVs, at approach to landing that provides high drag when the airbrake is deployed, without comprising lift, thereby allowing for relatively steep glide angles, although in such cases the standard speed safety margin V_(flight)>1.2V_(stall) is maintained.

The SA-MS aerofoil sections for the wing 10 can be designed in any suitable manner, for example based on the methods as disclosed in U.S. Pat. No. 8,109,473 or U.S. Pat. No. 7,992,827, mutatis mutandis, these references being incorporated herein in their entirety, and incorporating the axial overlap as disclosed herein.

For example, it is possible to start with a known slotted aerofoil design that can be close in characteristics to that required, for example camber, thickness to chord ratio, and so on. Alternatively, a baseline aerofoil can be designed using known methods. Then, the baseline aerofoil contour can be modified to provide a relatively blunt leading edge to obtain high lift, and a suction surface that is cambered and/or has a thickness distribution such as to provide slowly creeping trailing edge separation. Then, the profile of a downstream portion of the suction surface of the leading element of the slotted aerofoil is changed so as to provide a local curvature (1/r_(local)) that in some parts thereof increases, and/or in other parts thereof remains substantially constant; at the same time, the longitudinal overlap between the primary element and the secondary element is chosen according to the presently disclosed subject matter. CFD methods can be used, for example, to test the aerofoil, which can then be modified further, and again tested to determine whether the modification is providing the required aerodynamic effect. A number of such trial and error iterations can be carried out until a suitable profile for the aerofoil, providing the required characteristics, is achieved.

The full span of the half-wing 10 can comprise the same aerofoil profile (scaled if necessary according to the aspect ratio, for example), save possibly for aerodynamic or structural features such as booms, wing tip winglets and the like, for example, in a similar manner to the configuration of regular slotted aerofoils along the wing span of the IAI Heron, for example, mutatis mutandis. Alternatively, different portions of the half-wing 10 can comprise different aerofoil profiles, suitably blended along the span or optionally comprising a flow separation plate or the like separating the various wing portions.

In the method claims that follow, alphanumeric characters and Roman numerals used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps.

Finally, it should be noted that the word “comprising” as used throughout the appended claims is to be interpreted to mean “including but not limited to”.

While there has been shown and disclosed certain examples in accordance with the presently disclosed subject matter, it will be appreciated that many changes can be made therein without departing from the spirit of the presently disclosed subject matter. 

1. A method for operating an air vehicle, the air vehicle comprising fixed wings configured to provide mild stall characteristics including a post-stall regime, and a propulsion system capable of generating a controllable thrust, the thrust being variable at least between an idle thrust and a maximum thrust, the method comprising: during a landing maneuver, causing the air vehicle to attain an angle of attack corresponding to said post-stall regime; and during said landing maneuver, concurrently generating a thrust level of said thrust greater than said idle thrust to provide a thrust vector having a thrust lift force component at landing.
 2. The method according to claim 1, further comprising, at least during said landing maneuver, generating significantly elevated drag levels as compared with nominal drag levels prior to said landing maneuver.
 3. The method according to claim 1 or claim 2, wherein said angle of attack is greater than 10°.
 4. The method according to any one of claims 1 to 3, wherein said angle of attack is between 10° and 20°.
 5. The method according to any one of claims 1 to 4, wherein said thrust lift force component is between 10% and 50% of said thrust level.
 6. The method according to any one of claims 1 to 5, wherein said thrust level is between 10% and 50% of said maximum thrust.
 7. The method according to any one of claims 1 to 6, wherein in said landing maneuver, the air vehicle is in a downward glide path defined by a glide angle.
 8. The method according to claim 7, wherein said glide angle is in the range of between −3° to about −15°.
 9. The method according to any one of claims 7 to 8, wherein said thrust vector is at a pitch angle (θ) to the horizontal, wherein said pitch angle (θ) is related to said angle of attack (α) and said glide angle (γ) by the expression: α=θ−γ
 10. The method according to claim 9, wherein said thrust lift force component (TL) is related to said thrust level (T) and said thrust vector pitch angle (θ) by the expression: TL=T*sin θ
 11. The method according to claim 9 or claim 10, wherein said thrust vector has a forward thrust component (TH) at landing, wherein said forward thrust component (TH) is related to said thrust level (T) and said thrust vector pitch angle (θ) by the expression: TH=T*cos θ
 12. The method according to any one of claims 7 to 11, wherein said glide angle (γ) is related to said thrust level (T) and said elevated drag (D) by the expression: γ=(D−T)/L
 13. The method according to any one of claims 1 to 12, comprising operating the air vehicle during said landing maneuver at airspeeds below the corresponding speed safety margin, while the air vehicle is airborne.
 14. The method according to any one of claims 1 to 13, comprising operating the air vehicle during said landing maneuver at airspeeds not exceeding the corresponding stall airspeed of the air vehicle, while the air vehicle is airborne.
 15. The method according to any one of claims 1 to 14, comprising operating the air vehicle during said landing maneuver at airspeeds between 90% and 95% of the corresponding stall airspeed of the air vehicle, while the air vehicle is airborne.
 16. The method according to any one of claims 1 to 15, wherein said air vehicle is configured for providing said elevated drag levels while not significantly degrading aerodynamic lift provided by said wings during said landing maneuver.
 17. The method according to claim 16, wherein said fixed wings are lift generating wings having at least some aerofoil cross-sections thereof comprising a two element aerofoil, having an aerofoil chord, a primary element having a first leading edge and a first trailing edge, a secondary element having a second leading edge and a second trailing edge, a gap between the primary element and the secondary element, and an axial overlap between the first trailing edge and the second leading edge, the secondary element being deflectable with respect to the primary element about a fixed hinge point by a flap deflection angle, the secondary element being configured to operate in airbrake mode when deflected by a respective said flap deflection angle corresponding to a design airbrake deflection angle wherein to generate an airbrake drag, wherein at least for said design airbrake deflection angle said axial overlap is numerically greater than −0.5% of the aerofoil chord.
 18. The method according to claim 17, wherein said design airbrake deflection angle is greater than 40°.
 19. The method according to claim 17 or claim 18, wherein said design airbrake deflection angle is greater than 45°.
 20. The method according to any one of claims 17 to 19, wherein said design airbrake deflection angle is greater than 55°.
 21. The method according to any one of claims 17 to 20, wherein said airbrake drag corresponds to an airbrake aerofoil drag coefficient that is at least 150% greater than a datum drag coefficient of the aerofoil at a zero said flap deflection angle.
 22. The method according to claim 21, wherein said airbrake aerofoil drag coefficient is greater than 0.15.
 23. The method according to claim 21 or claim 22, wherein said airbrake aerofoil drag coefficient is greater than 0.2.
 24. The method according to any one of claims 21 to 23, wherein said airbrake aerofoil drag coefficient is greater than 0.3.
 25. The method according to any one of claims 17 to 24, wherein the primary element is configured for providing high lift mild stall characteristics, and wherein the aerofoil is configured to generate said airbrake drag while concurrently retaining said mild stall characteristics.
 26. The method according to any one of claims 17 to 25, wherein at least for said design airbrake deflection angle said axial overlap is between −0.5% and +4% of the aerofoil chord.
 27. The method according to any one of claims 17 to 26, wherein said axial overlap provides a smooth variation of aerofoil maximum lift coefficient with said flap deflection angle for a range of said flap deflection angles at least ±10° from said design airbrake deflection angle.
 28. The method according to any one of claims 17 to 27, wherein at said axial overlap, the aerofoil maximum lift coefficient is maintained constant within 0.1 with said flap deflection angle for a range of said flap deflection angles at least ±10° from said design airbrake deflection angle.
 29. The method according to any one of claims 17 to 28, wherein a value for said axial overlap is chosen to maximize the maximum lift coefficient obtained for the aerofoil at said design airbrake deflection angle.
 30. The method according to any one of claims 17 to 29, wherein said secondary element is in the form of a slotted flap.
 31. The method according to any one of claims 17 to 30, wherein the secondary element is deflectable with respect to the primary element about said fixed hinge point to provide said flap deflection angle ranging between about −15° and +80°.
 32. The method according to any one of claims 17 to 31, wherein at said design airbrake deflection angle, at least a majority of an airflow over said secondary element is fully detached.
 33. The method according to any one of claims 17 to 32, wherein at said design airbrake deflection angle said aerofoil generates a maximum lift coefficient of about 2.5 or greater than 2.5.
 34. The method according to any one of claims 17 to 33, wherein at said design airbrake deflection angle corresponds to a design Reynolds number of between 0.3*10⁶ and 1.0*10⁶.
 35. The method according to any one of claims 17 to 34, wherein at said design airbrake deflection angle corresponds to a design Reynolds number of 0.4*10⁶.
 36. The method according to any one of claims 1 to 35, wherein the air vehicle is a UAV.
 37. The method according to any one of claims 1 to 36, wherein the air vehicle has a weight between about 50 kg to about 500 kg.
 38. A control system for operating an air vehicle, the air vehicle comprising fixed wings configured to provide mild stall characteristics including a post-stall regime, and a propulsion system capable of generating a controllable thrust, the thrust being variable at least between an idle thrust and a maximum thrust, the control system comprising a controller configured for: causing the air vehicle to attain an angle of attack corresponding to said post-stall regime during a landing maneuver; and concurrently generating during said landing maneuver, a thrust level of said thrust greater than said idle thrust to provide a thrust vector having a thrust lift force component at landing.
 39. The control system according to claim 38, configured for operating according to the method defined in any one of claims 1 to
 37. 40. An air vehicle, comprising the control system as defined in any one of claims 38 to
 39. 